Ukip sources later offered an olive branch to Eurosceptic Tory MPs, who will be fearing that fighting a Ukip candidate will split their vote and let in a rival.

Mr Farage said that fighting all of the seats in 2015 gave the party the best possible opportunity to be left with some MPs and even hold the balance of power in a hung Parliament.

He said: "If you really want a referendum you make sure that Ukip win the Euro elections next year to keep the pressure on and then you make sure that Ukip has decent representation in the House of Commons in 2015.

Who knows? What if Ukip held the balance of power - there would be a referendum then."

They said that while Ukip’s leadership would be willing to turn a blind eye if local activists insisted that they could not stand against a Eurosceptic Tory MP.

Mr Farage said he still did not know if he would try to win an MP’s seat at the election. Mr Farage said: “I am focusing my energy on trying to turn the European elections next year into a referendum on our membership of the European Union.”

He added that he expected next May's European Union elections - which he said were "the most important since 1979" - to be “dominated” by the opening of the borders to migrants from Bulgaria and Romania in January next year.

Mr Farage renewed his calls for Britain to hold a referendum on EU membership before 2017, which has been pledged by Prime Minister David Cameron.

He said: “I think it is a terrible waste of time. Just how much more damage will be done to British business between now and then. It is unacceptable to be made to wait that long.”

Mr Farage said that between now and then Britain faced “four wasted years – there is no renegotiation on offer”.